I installed a new clutch slave. When I plugged in the supply line the cylinder leaked like a freakin sieve. Even with the cap on the reservoir fluid was running out of the connection into the cylinder.
I double-checked that the grommet was all the way in (heard it click) and I was able to push in the safety clip into it's normal position.
The only explination that I have is that the seal was compromised during the extraction/insertion. So unless anyone has a bright idea it looks like Im ordering a new hydraulic line.

Which seal? The one on the new clutch slave? Given you are replacing this component because of a failure, it might be expected to replace the line. By now you have probably solved it all, but what advice would you pass on?

The seal is actually part of the hydraulic line.
I'm not sure that I agree that one would expect to replace the line if the cylinder is bad. You don't replace your brake line when replacing a caliper.
Nevertheless it looks like that's what I'll be doing. This certainly turns a relatively easy job into a bigger PITA.
In my case I don't know that there was anything wrong with the slave. but I was flushing the hydraulics and wanted to replace it as a bit of preventive maintenance.

The real PITA is going to be when you bleed the new slave. This drives veteran mechanics crazy.

It's really not that hard if you do it right.
Start by referencing this video. I used a piece of 1/2 x 1 wood instead of the steel that they use in the video to make my little compressor tool.
The other thing that I think is pretty helpful is to bleed the cylinder before you mount it to the tranny. That way you can orient it so that the bleed nipple is higher than the supply line connection. I had zero problems doing it this way.